Appian now has two products that accomplish a quad-monitor setup. The first is the Appian Jeronimo Pro, which is the company's most poplular unit, and the Appian X, which is a brand new model that comes with DVI outputs and Appian's proprietary AGX chip, a much higher performing chip that the one found in the Jeronimo Pro.
Both models come with special adapter cables that enable four monitors to be connected. The adapter cables are necessary becaues its physically impossible to squeeze four VGA connectors on the face of a single PCI card.
I use the Jeronimo Pro myself running Compaq 18" flat panels and it works great. All monitors run at 1280 x 1024.